In-vehicle radar devices are well known. Such in-vehicle radar devices detect a position of each reflection point that reflects a radar wave, and the speed of each reflection point relative to the own vehicle, according to the transmission/reception signal of the radar wave (see PTL 1). The in-vehicle radar device described in PTL 1 determines whether each reflection point is a stationary object or a moving object on the basis of the speed of the reflection point relative to the own vehicle and the speed of the own vehicle.
In the in-vehicle radar device described in PTL 1, the weather is determined to be bad if the number of reflection points determined to be stationary objects is less than a criterion, which is the number of reflection points that have been determined to be stationary objects when the weather is fine.